The present invention relates generally to radio transmission systems, in particular mobile telephones, and more particularly to microstrip antennas included in such systems.
An antenna of this kind includes a patch which is typically formed by etching a metallic layer. The skilled person refers to an antenna of this kind as a xe2x80x9cmicrostrip patch antennaxe2x80x9d.
Microstrip technology is a planar technology which is used both to produce signal transmission lines and antennas which provide coupling between such lines and radiated waves. It uses conductive patches and/or strips formed on the upper surface of a thin dielectric substrate. A conductive layer extends over the bottom surface of the substrate and constitutes a ground plane for the line or the antenna. A patch of the above kind is typically wider than a strip of the above kind and its shape and dimensions constitute important characteristics of the antenna. The substrate is typically a plane rectangular sheet of constant thickness and the patch is also typically rectangular. This is no way obligatory, however. In particular, the skilled person knows that varying the thickness of the substrate can increase the bandwidth of an antenna of the above kind and that the patch can take various shapes, for example it can be circular. The electric field lines extend through the substrate between the strip or the patch and the ground plane.
The above technology differs from various other technologies which also use conductive members on a thin substrate, and in particular it differs from coplanar line technology in which the electric field is established over the upper surface of the substrate and in a symmetrical manner between a central conductive strip and two conductive areas on respective opposite sides of the strip, from which they are respectively separated by two slots. In the case of a loop slot antenna, a patch is surrounded by a continuous conductive area from which it is separated by a slot.
Antennas constructed using the above technologies typically, although not necessarily, constitute resonant structures in which standing waves provide coupling with waves radiated into space.
Various types of resonant structure can be implemented using the microstrip technology and can employ various modes of resonance, which modes are referred to more briefly hereinafter as xe2x80x9cresonancesxe2x80x9d. Broadly speaking, each such resonance can be described as a standing wave formed by the superposition of two traveling waves propagating in two opposite directions on a common path, the two waves resulting from the same traveling electromagnetic wave being reflected alternately at each of the two ends of the path. In the context of a description of this kind, the wave is considered to propagate in an electromagnetic line comprising the ground plane, the substrate, and the patch, and which defines a linear path of zero width. In fact a wave of the above kind has wavefronts which extend transversely across the whole of the section offered to them by the antenna, which means that the above description simplifies the real-life situation in a manner that is sometimes excessive. To the extent that it can be considered linear, the path can be rectilinear or curved. It is referred to hereinafter as the xe2x80x9cresonance pathxe2x80x9d. The resonant frequency is inversely proportional to the time taken by the above-mentioned traveling wave to travel along this path.
A first type of resonance might be referred to as xe2x80x9chalf-wavexe2x80x9d resonance. In this type of resonance the length of the resonance path is typically substantially equal to one half-wavelength, i.e. to half the wavelength of the traveling wave referred to above. The antenna is then referred to as a xe2x80x9chalf-wavexe2x80x9d antenna. This type of resonance can generally be defined by the presence of an electric current node at each of the two ends of a path of the above kind, whose length can therefore be equal to said half-wavelength multiplied by an integer other than one. This integer is typically odd. Coupling with the radiated waves occurs at at least one of the two ends of the path, which ends are in regions where the amplitude of the electric field in the substrate is at a maximum.
A second type of resonance that can be obtained using the same technology might be referred to as xe2x80x9cquarter-wavexe2x80x9d resonance. It differs from said half-wave resonance firstly in that the resonance path typically has a length substantially equal to one fourth of a wavelength, i.e. one quarter of the wavelength as defined above. For this the resonant structure must have a short-circuit at one end of the path, the expression xe2x80x9cshort-circuitxe2x80x9d referring to a connection between the ground plane and the patch. The short-circuit must have a sufficiently low impedance to be able to impose such resonance. This type of resonance can be generally defined by the presence of an electric field node fixed by a short-circuit of the above kind in the vicinity of if an edge of the patch and by an electric current node at the other end of the resonance path. The length of the resonance path can therefore be equal to an integer number of half-wavelengths added to said quarter-wavelength. Coupling with the waves radiated into space occurs at an edge of the patch, in a region where the amplitude of the electric field through the substrate is sufficiently high.
Other types of more or less complex resonance can be established in antennas of the above kind, each resonance being characterized by a distribution of the electric and magnetic fields, which fields oscillate in an area of space including the antenna and its immediate vicinity. They depend in particular on the configuration of the patches, which can in particular incorporate slots, possibly radiating slots. They also depend on the possible presence and location of short-circuits and electrical models representing those short-circuits when they are imperfect short-circuits, i.e. when they cannot be treated, even approximately, as equivalent to perfect short-circuits with zero impedance.
The presence of an imperfect short-circuit in an antenna can give rise to resonance featuring what might be referred to as a virtual node. A virtual node is produced if some of the following conditions are satisfied at the same time. If the above antenna is referred to as the xe2x80x9cfirst antennaxe2x80x9d, these conditions are as follows:
The distribution of the fields in the first antenna is substantially identical to a distribution that can be induced in an identical area of the patch of a second antenna.
The second antenna is identical to the first antenna within this area except that within this area the second antenna does not have the short-circuit in question.
The patch of the second antenna extends not only over the area already mentioned, which then constitutes a main area of the second antenna, but also over a complementary area.
Finally, the distribution of the fields in question in the main area of the second antenna is accompanied by an electric or magnetic field node in the complementary area.
When describing the resonance occurring in the first antenna, the node occurring in the second antenna may be considered also to constitute a node for the resonance of the first antenna. For an antenna such as the first antenna, a node of this kind is referred to hereinafter as a xe2x80x9cvirtualxe2x80x9d node, because it is in an area which is outside the patch of the antenna and in which no electric or magnetic field therefore occurs whereby the presence of the node could be determined directly.
Although these xe2x80x9cvirtual nodesxe2x80x9d are not conventionally taken into account in these terms in describing resonances, they are implicit in the distinction that is sometimes drawn between the physical or geometrical length and the so-called electrical length of the same patch. In the case of two antennas referred to above, and with regard to the patch of the first antenna, the physical or geometrical length would be that of the patch and the electrical length of the same patch would in fact be the physical or geometrical length of the second antenna.
More than one resonance can occur for a given antenna configuration. They then enable the antenna to be used at each of the resonant frequencies.
An antenna is typically coupled to a signal processor such as a transmitter via a connection system including a connection line which is external to the antenna and terminates at a coupling system integrated into the antenna for coupling the line to a resonant structure of the antenna. The resonances of the antenna also depend on the nature and on the location of this system. In the case of transmit antennas, the connection system is often referred to as a feed line of the antenna.
The present invention relates to various types of systems such as mobile telephones, base transceiver stations for mobile telephones, automobiles and aircraft or airborne missiles. In the case of a mobile telephone, the continuous nature of the bottom ground plane layer of a microstrip antenna readily limits the radiation intercepted by the body of the user of the system. In the case of automobiles, and above all in the case of aircraft or missiles, whose outside surface is of metal and has a curved profile producing very low aerodynamic drag, the antenna can be conformed to that profile so as not to generate any unwanted additional aerodynamic drag.
The invention relates more particularly to the situation in which a microstrip antenna must have the following qualities:
it must be a two-frequency antenna, i.e. it must be able to transmit and/or receive efficiently radiated waves at two frequencies separated by a large spectral spacing,
it must be connectable to a signal processor by a single connection line for all operating frequencies of a transmission system without giving rise in that line to any unwanted and troublesome standing wave ratio, and
it must not be necessary to use a frequency multiplexer or demultiplexer to achieve the above.
Many microstrip antennas have been constructed or proposed in the art which have the above three qualities. They differ from each other in terms of the means employed to obtain a plurality of resonant frequencies. Three such antennas are discussed below:
A first such prior art antenna is described in U.S. Pat. No. 4,766,440 (Gegan). The patch 10 of that antenna is of generally rectangular shape, enabling the antenna to exhibit two half-wave resonances whose paths are established along a length and across a width of the patch. It also has a U-shaped curved slot which lies entirely inside the patch. The slot is a radiating slot and produces an additional mode of resonance with another path. By an appropriate choice of its shape and dimensions, it also tunes the frequencies of the resonance modes to required values, which offers the facility to transmit a circularly polarized wave by associating two modes having the same frequency and crossed linear polarization. The coupling system takes the form of a microstrip line, but the line can also be said to be coplanar in that the microstrip is in the plane of the patch and penetrates between two notches therein. The system includes impedance matching means for matching it to the various input impedances of the line at the various resonant frequencies used as operating frequencies.
That first prior art antenna has the following drawbacks in particular:
The need to provide impedance matching means makes it complicated.
Accurate adjustment of the resonant frequencies to required values is difficult.
A second prior art antenna differs from the previous one in that it uses only one resonance path. It is described in U.S. Pat. No. 4,771,2 (LO et al.). Its patch includes localized short-circuits and slots extending along straight line segments inside the patch. The slots and short-circuits reduce the difference between two frequencies corresponding to two resonances which share said path but have two respective and mutually different modes which are designated by the numbers (0,1) and (0,3), i.e. the common path is occupied by one half-wave or by three half-waves according to the mode concerned. The ratio between these two frequencies can therefore be reduced from 3 to 1.8. The localized short-circuits are formed by conductors passing through the substrate.
That second prior art antenna has in particular the drawback that its fabrication is complicated by the inclusion of localized short-circuits.
A third prior art two-frequency antenna differs from the preceding antennas in that it uses quarter-wave resonance. It is described in the article: xe2x80x9cDual Band Cavity-Backed Quarter-wave Patch Antennaxe2x80x9d by Boag et al. published in IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM DIGEST, NEWPORT BEACH, JUN. 18-23, 1995, pages 2124-2027. A first resonant frequency is defined by the dimensions and the characteristics of the substrate and the patch of the antenna. A resonance of substantially the same type is obtained at a second frequency on the same resonant path by using a matching system.
That third prior art antenna has the following drawbacks in particular:
The difference between the two resonant frequencies is too small for some applications.
The need to use a matching system makes the antenna complicated.
The same can apply to using an antenna coupling system in the form of a coaxial line.
The present invention has the following objects in particular:
to simplify implementing a two-frequency antenna,
to enable a freer choice than previously of the ratio of the center frequencies of two operating bands of a transmission system, and more particularly of providing an antenna for this system such that the ratio of two wanted resonant frequencies of the antenna is from approximately 1.25 to approximately 5, and in particular close to 2,
to give the antenna a bandwidth centered on each of the two resonant frequencies which is sufficiently large to enable a transmit frequency and a receive frequency of the system to be situated in each of the two bands without causing crosstalk,
to enable easy and accurate adjustment of the two resonant frequencies,
to enable the use of a single coupling system whose impedance can easily be matched for each of the two resonant frequencies, and
to limit the dimensions of the antenna.
With the above objects in view, the invention provides in particular a two-band transmission system including:
a signal processor adapted to be tuned in two operating bands centered on respective predetermined center frequencies to transmit and/or receive an electrical signal in each of the two bands,
a microstrip antenna, and
an antenna connection system including electrical conductors connecting the processor to the antenna for coupling said electrical signal to radiated waves, wherein the antenna includes:
a conductive ground plane,
a conductive patch having a periphery,
a short-circuit formed at said periphery, and
a separator slot having an origin consisting of an opening in said periphery, said slot penetrating said patch from said origin,
wherein said short-circuit and said separator slot enable two resonances to be established in said antenna, one of said two resonances is of the quarter-wave type with an at least virtual electric field node fixed by said short-circuit, constitutes a primary resonance and has a primary frequency substantially equal to one of said two center frequencies, and the other of said two resonances constitutes a secondary resonance having a secondary frequency substantially equal to the other of said two center frequencies,
and wherein said electrical conductors of the connection system include said ground plane and a main antenna coupling conductor which is part of said patch to enable said antenna to be coupled to said signal processor around each of said two center frequencies,
which transmission system is characterized in that said separator slot extends in said patch as far as a rear end of said slot located at a sufficiently small distance from said periphery for said slot to divide said patch partly into a body including said main antenna coupling conductor and said short-circuit and a tail free of said short-circuit and electrically connected to said connection system only by means of said body and a passage consisting of an area of said patch between said back and said periphery.
The separator slot is preferably at a distance from the periphery of the patch greater than the distance from the rear end of the slot to the periphery over all of its length except in the vicinity of its origin or at least over a major portion of its length and on both sides.
The origin of the separator slot is preferably near said short-circuit so as to confer on said two resonances respective resonance paths which both extend from said short-circuit, one of said two paths preferably extends only in said body and the other of said two paths preferably extends in said body and in said tail.